Almost all global severe weather occurs in the middle latitudes between approximately 30° and 65° of latitude. Cyclonic storms, blizzards, severe thunderstorms and tornadoes are created where warm and cold air meets and that is most dramatic along what is generally known as the Polar Front. This world map of tornado zones illustrates the point. 

Source: ncdc.noaa.gov

Here is a simplified diagram of the division between the cold polar air and the warm tropical air.

Source:  Source: Fundamentals of Physical Geography, Briggs, Smithson, Ball et al..

Temperature contrast across the Polar Front is the greatest in a short distance in each hemisphere. This creates the strongest winds as illustrated by the location of the Jet Stream (more correctly called the Circumpolar Vortex) above the surface. It also means the formation of swirling low pressure systems or cyclones that in winter are blizzards.  As the cold air advances it pushes up unstable bubbles of warm air to create heavy rain from large clouds. With enough force these can develop in to severe thunderstorms (cumulonimbus) and under certain conditions trigger tornadoes. These conditions occur most frequently across the central US in what is colloquially known as Tornado Alley.

Source: severewx.atmos.uiuc.edu

1. Frequency and intensity of most severe weather is a direct function of the temperature contrast across the Polar Front. This spring the cold air stayed further south with the colder temperatures with the resulting severe weather and flooding across the central US.

2. IPCC Reports claim increased CO2 levels will make the Polar air warm more than the tropical air. If true, this will decrease the temperature contrast across the Front resulting in fewer storms and less severe weather.

Source: ncdc.noaa.gov

3. The graph from the National Oceanographic and Atmospheric Administration (NOAA) shows that severe tornadoes were higher in the period from 1950 to 1975.  Global temperatures were falling during that time. Since then frequency has decreased as the world warmed to 2000. Since then the world has cooled slightly and the pattern shows a slight increase in severe tornadoes.